Enantioselective deacylation

Enantioselective deacylation of esters in micellar reactions has been extensively studied in order to understand enzyme stereospedficity, and some micellar systems... 

Table 12. Enantioselectivities in the acylation and deacylation steps in the burst kinetics of the reaction of (Z)-Phe-PNP(52)...

The ratios of these slopes for L- and D-esters are shown in Table 12. The kL/kD values of the acylation step in the CTAB micelle are very close to those in Table 9, as they should be. It is interesting to note that the second deacylation step also occurs enantioselectively. Presumably it is due to the deacylation ocurring by the attack of a zinc ion-coordinated hydroxide ion which, in principle, should be enantioselective as in the hydroxyl group of the ligand. Alternatively, the enantioselectivity is also expected when the free hydroxide ion attack the coordinated carbonyl groups of the acyl-intermediate with the zinc ion. At any rate, the rates of both steps of acylation and deacylation for the L-esters are larger than those for the D-esters in the CTAB micelle. However, in the Triton X-100 micelle, the deacylation step for the D-esters become faster than for the L-esters. 

Using this approach, racemates of (27) were enantiomerically enriched using a lipase in organic solvent, followed by racemization of the unreacted enantiomer in buffer. Acylated derivatives (S)-(28) were obtained in yields >50% and >99% ee. Lipases with the opposite enantioselectivity produced (R)-28 in >99% ee. Subsequent chemical deacylation of (28) yielded enantiomerically enriched (27). 

Isocyanide polymers functionalized with amino acid groups, typically di-or tripeptides containing histidine or serine, give enantioselective deacylation and rate enhancements. Their activity is increased by addition of cationic surfactants (Visser et al., 1985). 

Deacylation of p-nitrophenyl esters of amino acids Histidine-functionalized micelles and synthetic vesicles. Rate enhancements and enantioselectivity observed Murakami et al., 1981 b... 

Deacylation of p-nitrophenyl esters of amino acids Synthetic vesicles + histidine-functionalized surfactant. Cholesterol increased rates and enantioselectivity Ueoka and Matsumoto, 1984... 

Deacylation of p-nitrophenyl derivatives of amino acids Chiral histidine-derived surfactants + CTABr. Rates and enantioselectivities examined Matsumoto and Ueoka, 1983... 

Deacylation or hydrolysis of chiral carbamates, carbonates and alkanoates Micelles and comicelles of N-hexadecyl-N-methylephedrinium bromide or N -myristoyl-histidine with CTABr. Rate effects and enantioselectivities examined Fomasier and Tonellato, 1984... 

The solvent present in biphasic reactions can still have an effect on the enzyme even though the enzyme functions primarily in an aqueous microenvironment. A particularly dramatic example is the lipase AH (lipase from Burkholderia cepac/fl)-catalysed desym-metrization of prochiral 1,4-dihydropyridine dicarboxylic esters, where either enantiomer can be accessed in high enantioselectivity by using either water-saturated cyclohexane or diisopropyl ether (DIPE) respectively (Scheme 1.60). The acyl group used in acylation and deacylation can also have a dramatic effect on enantioselectivity. " ... 

This process has many benefits in the context of green chemistry it involves two enzymatic steps, in a one-pot procedure, in water as solvent at ambient temperature. It has one shortcoming, however-penicillin acylase generally works well only with amines containing an aromatic moiety and poor enantioselectivities are often observed with simple aliphatic amines. Hence, for the easy-on/easy-off resolution of aliphatic amines a hybrid form was developed in which a hpase [Candida antarctica hpase B (CALB)] was used for the acylation step and peniciUin acylase for the deacylahon step [22]. The structure of the acyl donor was also optimized to combine a high enanhoselectivity in the first step with facile deacylation in the second step. It was found that pyridyl-3-acetic acid esters gave optimum results (see Scheme 6.8). 

Although many publications have covered the enantioselectivity of lipases in the deacylation step, their enantioselectivity in the acylation step (i.e., towards the acyl donor) has received much less attention. Generally, the selectivity of lipases towards racemic esters or acids is low to moderate [75-77]. Directed evolution and site-directed mutagenesis lead to a significant increase in the selectivity of the wild-type enzymes [78-80]. However, the enantiomeric ratios attained are still well below those typically obtained in kinetic resolutions of secondary alcohols. 

For resolution of the racemate 12 two different procedures can be applied 124 the en-antioselective enzymatic deacylation of chloroacetyl-DL-a-aminosuberic acid at pH 7.2 with Taka-acylase or the enantioselective salt precipitation of Z-dl-Asu-OH with D-tyrosine hydrazide according to the method of Vogler et alJ25 Complete enzymatic digestion is achieved in about ten days at 37 °C, and the optically pure L-enantiomer is obtained in 80% yield but the overall efficiency is lower than that of the chemical method. Fractional crystallization affords in good yields the Z-l-Asu-OH derivative 13 which is then used directly as a suitably protected intermediate in subsequent derivatization steps (see Scheme 6). Moreover, the recovery of the D-enantiomer from the mother liquors is also easy in this case. 

Enzymes are widely recognized as valuable tools for the synthesis of optically active compounds [22]. Thus, lipase-catalyzed acylation or deacylation is one of the most efficient methods for the preparation of optically active alcohols, acids, and esters. Because lipases retain activity and selectivity in non-conventional media such as organic liquids, their use as biocatalysts in enantioselective synthetic reactions has considerably increased. 

All three isomerizations discussed above seem to occur by analogous mechanistic pathways similar to the mechanism formulated for the Dakin-West reaction [82]. Deacylation of the starting material H by catalyst G affords, in a fast and reversible step (Scheme 13.47, step I), an acylpyridinium/enolate ion-pair I. From this ion pair, enantioselective C-acylation proceeds in the rate-determining and irreversible second step, furnishing the C-acylated product J (Scheme 13.47, step II). 

In order to broaden the scope we also examined [30] a combination of lipase-catalyzed acylation with penicillin acylase-catalyzed hydrolysis (deacylation). Good results (high enantioselectivity in the acylation and smooth deacylation) were obtained, with a broad range of both aliphatic amines and amines containing an aromatic moiety, using pyridylacetic acid ester as the acyl donor (Fig. 9.21). 

Enantioselective lipase-catalyzed transesterification involving deacylation of esters of racemic primary or secondary alcohols with primary alcohols, most frequently -butanol, serving as an acyl acceptor, is fairly common. Recent examples include esters of amino alcohols, isoserine, chlorohydrins, and various to-syloxybutanoate esters (eq 8). ... 

Bayer improved upon this concept with the selective hydrolysis of racemic acetamide 3 using C. antarctica lipase B (Figure 14.3) [6]. In this case, the (1 ) acetamide was deacylated, forming the (U) amine 4. Although the enantioselectivities obtained were excellent, the process was not used industrially due to low space time yields. 

In the Novozym 435 catalysed ring-opening of a (chiral) substituted lacton, both acylation and deacylation can be enantioselective. For example, it is well known that CALB shows pronounced selectivity for / -secondary alcohols in the deacylation step. Since the forward and backward reaction exhibits by definition the same selectivity, esters comprising a substituent at the alcohol side are expected to show pronounced / -selectivity in the acylation step. " This is indeed observed for 7-MeHL, 8-MeOL and 12-MeDDL (Table 1). However, the selectivity for acyl donor in the case of PBL, 5-MeVL and 6-MeCL -lactones in which the ester bond is exclusively in a cisoid conformation- is low or for the S-enantiomer. We can speculate that lactones in a cisoid conformation must attain a different orientation in the active site in order to be activated. ... 

Recently, we exploited the unique spectroscopic properties of AW to probe the disulfide-coupled folding of the hirudin N-terminal domain 1-47 and the binding to its the target enzyme, thrombin [90]. Before chemical synthesis, the resolution of the commercially available enantiomeric mixture of AW was carried out by treating the racemic mixture with acetic anhydride and subsequent enantioselective deacylation with immobilized Asperigillus oryzae acylase-I to yield L-AW (Scheme 9-1). Purified L-AW was then reacted with 9-fluorenylmethoxycarbonyl chloride... 

An enantioselective route to 1,3-dithiane 1-oxide (33) (R = R = H) was subsequently developed [69]. It involves asymmetric oxidation of (32) (R = pivaloyl, R = H) by cumene hydroperoxide in presence of the chiral titanium complex. The syn/anti mixture (around 90% ee for each diastereoisomer) is recrystallized and then deacylated, giving the desired product in 80% yield. A recent application of this chemistry is the asymmetric synthesis of enantiopure (R)-(-)-2,6-dimethylheptanoic acid in two steps from (33) (R = C(0)Et, R = Et) [70]. The reaction involves a fully stereoselective methylation in the a-position of the keto group, followed by basic deacylation, which also regenerates enantiopure 2-ethyl-l,3-dithiane 1-oxide (33) (Ri = H, R = Et). A range of a-arylpropanoic acids have since been prepared by similar routes in high ee s. [162]... 

Ohara and co-workers reported that the CALB-catalysed polycondensation of alkyl esters of lactic acid as the monomer produces oligoLAs (X = alkyl, n=2-7 in Scheme 12.1) [13].The reaction is perfectly enantioselective only the alkyl D-lactate monomer produced the oligomers. These results provide the first direct evidence that in the lipase-catalysed reaction mechanism the enantioselection is governed by the deacylation step of lipase . 

Synthetic highlights The partial synthesis of paclitaxel was necessary to enhance the availability of the drug substance and avoid unsustainable use of yew trees. Many different synthetic routes have been reported and three inventive pathways for the enantioselective or site-selective approaches to various segments of the paclitaxel molecule are described. These are aU promoted by organometal catalytic complexes. Reactions presented include use of the intramolecular Heck reaction in the synthetic pathway to baccatine III the Sharpless reaction and the introduction of a trifunctional catalyst for biomimetic synthesis of chiral diols synthesis of the paclitaxel side-chain and use of a Zr-complex catalyst in the reductive N-deacylation of taxanes to primary amine, the key precursor of paclitaxel. 

These findings led to elucidation of the mechanistic aspects of Upase (Novozym 435) catalysis enantioselection is operated by the deacylation step as shown in Fig. 3 [53], where only dimer formation is shown for simphcity. It is well accepted that at first the monomer (substrate) is activated by enzyme with formatimi of an (/ )-acyl-enzyme intermediate (enzyme-activated monomer, EM) [ acylation of lipase step (a) in Fig. 3]. Onto the activated carlxMiyl carbon of EM, the OH group of the D-lactate nucleophUically attacks to form an ester bond, liberating Upase enzyme and giving rise to D,D-dimer [ deacylation of Upase step (b) in Fig. 3]. 

Hydrolysis reaction of ethyl d- and L-lactates (EtLa)s catalyzed by protease were studied EtLLa was consumed a little faster than EtDLa. The mechanism of the protease-catalyzed oligomerization was similar to that of lipase (as seen in Figs. 3 and 4), but in an L-selective manner the enantioselection is governed by the deacylation step. 

Ohara H, Onogi A, Yamamoto M, Kobayashi S (2010) Lipase-catalyzed oligomerization and hydrolysis of alkyl lactates direct evidence in the catalysis mechanism that enantioselection is governed by a deacylation step. Biomacromolecules 11 2008-2015... 

Ohara H, Nishioka E, Yamaguchi S, Kawai F, Kobayashi S (2011) Protease-catalyzed oligomerization and hydrolysis of alkyl lactates involving L-enantioselective deacylation step. Biomacromolecules 12 3833-3837... 

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